The present invention relates to the field of solid oxide fuel cells, and more particularly, relates to a solid oxide fuel cell having vias and an improved interconnect made from a metal/ceramic composite material.
A fuel cell is a device in which a first reactant, a fuel such as hydrogen or a hydrocarbon, is electrochemically reacted with a second reactant, an oxidant such as air or oxygen, to produce a DC electrical output. A fuel cell includes an anode, or fuel electrode, which enhances the rate at which electrochemical reactions occur on the fuel side. There is also a cathode, or oxidant electrode, which functions similarly on the oxidant side. In the solid oxide fuel cell (hereafter SOFC), a solid electrolyte, made of, for example, dense yttria-stabilized zirconica (YSZ) ceramic separates a porous ceramic anode from a porous ceramic cathode. The anode is made of, for example, nickelous oxide/YSZ cermet, and the cathode is made of, for example, doped lanthanum manganite.
In such an SOFC, the fuel flowing to the anode reacts with oxide ions to produce electrons and water, which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and thence to the cathode. The electrolyte is a nonmetallic ceramic that is a poor or nonconductor of electrons, ensuring that the electrons must pass through the external circuit to do useful work. However, the electrolyte permits the oxide ions to pass through from the cathode to the anode.
Each individual electrochemical cell, made of a single anode, a single electrolyte, and a single cathode, generates a relatively small voltage. To achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. The cells are connected in series electrically in the stack. The fuel cell stack includes an electrical interconnect between the cathode and the anode of adjacent cells. The fuel cell assembly also includes ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
Numerous publications describe conventional SOFC which completely oxidize methane to carbon dioxide and water. These SOFC are not designed to conduct chemical processes, but rather to generate electricity from fuel gas and air (or oxygen). The processes conducted in SOFC are selected for complete combustion rather than partial combustion and require completion of an external electric circuit or oxidation of fuel gas for continuous operation.
The typical SOFC comprises an anode made of a mixture of nickel metal and YSZ and runs at 800-1000.degree. C. since internal reforming is most efficient at these high temperatures. However, the trend in SOFC is to lower the operating temperature of the SOFC to 550-800.degree. C. so that less exotic materials can be used for interconnects, electrical connections and materials of construction for the housing of the SOFC.
The ideal fuel for the anode is hydrogen but dangers of flammability, storage and energy storage density complicate its use. More commonly, the fuels used can be light hydrocarbons such as methane, propane, ethanol and methanol. Heavier fuels such as JP8 (jet fuel) and kerosene can also be used but in some cases the internal reforming is not efficient enough to reform the fuel and carbonaceous material is built up in the anode. Water vapor is typically added to the fuel source to aid reforming.
Various solutions have been proposed to improve the material properties of the interconnect. Seto et al., "CERMET FOR INTERCONNECTION OF SOFC", The Electrochemical Society Proceedings, vol. 93 (1993), the disclosure of which is incorporated by reference herein, discloses a cermet of 60 vol. % alumina and 40 vol. % Inconel 600 as an interconnect which has a thermal coefficient of expansion close to that of the zirconia electrolyte. However, the thermal coefficient of expansion of alumina is too high compared to zirconia.
Yoshimura et al. U.S. Pat. No. 5,279,906, the disclosure of which is incorporated by reference herein, discloses the flame spraying of nickel/chromium alloy powders plus alumina onto a carrier to make an interconnect. Only discrete layers can be made by this process.
Lockhart et al. U.S. Pat. No. 5,261,944, the disclosure of which is incorporated by reference herein, discloses a nickel/zirconia cermet for an anode. Nickel will tend to oxidize in oxidizing atmospheres leading to volumetric changes and consequent stress cracking.
Others have proposed SOFC arrangements having vias. Rohr U.S. Pat. No. 5,063,122, the disclosure of which is incorporated by reference herein, discloses an SOFC arrangement wherein the fuel and oxidant are distributed throughout the SOFC by vias. The disclosed SOFC requires multiple sintering operations to seal the distribution channel side walls and post-sintering machining to open the vias after sealing.
Meachem WO 93/17465, the disclosure of which is incorporated by reference herein, discloses discretely built layers which are then combined--not cofired.
Accordingly, it is a purpose of the present invention to have an improved SOFC having an interconnect with a cermet material.
It is a further purpose of the present invention to have an improved SOFC with vias for greater fuel and oxidant distribution efficiency.
It is yet another purpose of the present invention to have an improved process for the making of co-fired SOFC.